Recently, many devices include infrared communication functions. Infrared communications use light and therefore have high directivity. Accordingly, communications are possible only when light transmitted from a light-emitting section of a communication device at least has light intensity which allows a light-receiving section of another communication device to receive the light in a surveyable range without any obstacles. For that reason, a communication distance and a light-receiving angle of the light communication depend on transmission light intensity and reception light sensitivity.
An example of a standard for specifying a method of infrared communications is IrDA (Infrared Data Association). IrDA standard defines that transmission light intensity is 100 mW/sr or more for a standard device, and reception sensitivity is 10 μW/cm2 or less for the standard device. This allows communications with a distance of 1 meter and an angle of ±15 degrees between the communication devices.
However, it is sometimes requested that optical communications with the above standard are carried out with a light-reception angle of ±45 degrees or more and a distance of 2 meters or more between a mobile device and an installed device (e.g. TV, PC, DVD/HDD recorder, and printer).
In order to extend a communication distance and to widen a light-reception angle, there is a method for increasing transmission light intensity of a communication device. However, in order to obtain a light output of 100 mW/sr specified in IrDA standard, an LED consumes an electric current of approximately 400 mA or more.
For example, in order to extend a communication distance twice, four times as large light output which is a square of a distance ratio is required. Consequently, an LED consumes an electric current of 1.6 A or more, which is a very large amount. This is not an amount of an electric current which can flow in a mobile device driven by a buttery or a device connected via an electricity-saving AC adaptor.
Furthermore, if the large amount of an electric current flows, more amount of heat is generated, and accordingly a protection against the heat is required. Consequently, the communication device must have a structure against the heat, resulting in a larger size. When a smaller device is required in the case of mobile phones, a communication device in a larger size is against the original intention.
As described above, it is difficult to increase transmission light intensity of a communication device, and therefore it is necessary to increase reception sensitivity of the communication device. In order to extend a communication distance and to widen a light-reception angle, there is a technique to provide a lens of large diameter.
However, when the baud rate of optical communications is high, it is requested that a photodiode or a phototransistor is small or a large bias voltage is applied on the photodiode or the phototransistor. Normally, applying a large bias voltage leads to cost up of a whole system.
For that reason, a small photodiode or a small phototransistor is mounted for high-speed communications. However, in a case of a lens of large diameter, light flux incident to the lens from all directions is required to be converged on the photodiode or the phototransistor, which makes an optical design very troublesome.
On the other hand, another method for extending a communication distance and widening a light-reception angle is as follows: optical design for narrowing a light-reception angle is carried out so as to extend a communication distance, and a plurality of optical communication devices are mounted so as to widen a light-reception angle. The following technique is disclosed as a method for mounting a plurality of optical communication devices.
For example, Japanese Unexamined Patent Publication No. 11150/1993 (Tokukaihei 05-11150; published on Jan. 19, 1993) (Document 1) discloses a technique in which: electric signals converted from optical signals received by optical communication devices are subjected to a logical disjunction, and an electric signal obtained by the logical disjunction is regarded as a reception signal. The electric signal obtained by the logical disjunction is supplied to a signal process circuit and is demodulated.
Furthermore, Japanese Unexamined Patent Publication No. 98435/1998 (Tokukaihei 10-98435; published on Apr. 14, 1998) (Document 2) discloses a technique in which: light-receiving sections of optical communication devices which receive transmission optical signals are switched by a switch so that a light-receiving section is selected. A reception signal supplied via the selected light-receiving section is demodulated.
Furthermore, Japanese Unexamined Patent Publication No. 535873/2002 (Tokukai 2002-535873; published on Oct. 22, 2002) (Document 3) discloses a technique in which: optical signals supplied from light-receiving sections of optical communication devices are converted into digital signals, synchronization signals called preambles positioned at the head of the digital signals for PLL synchronization are used to detect changes in pulse width or pulse position of the preambles, thereby determining and comparing measurement indicative of intensity of the supplied optical signals. As a result of the comparison, a digital signal having measurement expected to be appropriate for a subsequent process is selected.
However, Document 1 has a problem as follows: in a case of a sufficiently slow optical signal, demodulation of the electric signal subjected to the logical disjunction can be carried out without any problem. However, assume a case where an optical signal converted from an original signal is a fast signal having more than several Mbps. At that time, out of electric signals converted from optical signals, an electric signal supplied from an optical communication device to which particularly weak light is supplied has a large jitter depending on intensity of light incident to optical communication devices.
The electric signal having the jitter is subjected to the logical disjunction, resulting in an electric signal having so large jitter that a signal processing circuit cannot process the jitter. Consequently, demodulation of the electric signal becomes difficult. As a result, communications are allowed only in an area narrower than the actual communicable area.
Furthermore, in Document 2, a transmission request signal is sent to a transmitter so as to switch light-receiving sections of optical communication devices. This results in communication loss. Furthermore, as light-receiving sections are switched by confirming the direction of the transmitter in accordance with a response signal from the transmitter, when the transmitter moves during communications, the communications may be broken down.
Furthermore, in Document 3, preambles are used not only for PLL synchronization, but also for stabilization of reception sensitivity of a photodiode or a phototransistor. Consequently, a pulse width or a pulse position of the preambles change regardless of light intensity, and accordingly it is impossible to judge whether the change is due to light intensity or not.
When light intensity is judged after stabilization of the change, a plurality of pulses are required. Consequently, it is uncertain whether pulses can be supplied sufficiently for PLL synchronization. On the other hand, it is possible to lengthen a preamble. However, this causes reduction of an effective band of communications and requires all transmitters to be designed to be capable of lengthening a preamble. For that reason, in reality, lengthening of a preamble is not carried out.